The invention was conceived as a solution to the problem of determining the integrity of deformed or otherwise damaged structures. It is equally applicable to determine the integrity of metallic materials subjected to nuclear radiation. The capability of the invention to measure accurately service-related changes in several mechanical properties has been demonstrated for both unirradiated and irradiated materials.
In order to assess the integrity of metallic structures following accidents or severe service conditions, knowledge of the material's mechanical properties (particularly flow properties and fracture toughness), the size and extent of induced defects or cracks, and the current thickness and residual stresses is required. This knowledge is particularly important for nuclear components such as pressure vessels and their supports because of the radiation-induced embrittlement.
In the case of derailed railroad tank cars carrying hazardous materials, the eventual cleanup operation requires assurance that moving and removing the derailed/damaged tank cars do not cause safety problems. In such a situation, the damage may cause the mechanical and physical properties to vary greatly over short distances, and accurate determination of these properties is needed to assess the structural integrity of damaged/deformed components. The other essential information: the size and extent of service- or damage-induced defects, and the current thickness (since appreciable decrease of material thickness reduces the load-carrying capability of the structure), can be determined by nondestructive test techniques such as ultrasonic inspection.
Presently the change in flow properties and fracture toughness might be obtained by performing tensile and fracture toughness tests on similar materials with simulated laboratory deformation/damage. Although this approach may be satisfactory in some instances, there are other circumstances where it might not be acceptable due to unknown component material or heat treatment condition. Furthermore, the applicability of results from simulated tests to determine the integrity of the actual deformed structure will always have some degree of uncertainty, and such an approach is very expensive. An article entitled "The Use of Miniaturized Tests to Predict Flow Properties and Estimate Fracture Toughness in Deformed Steel Plates" by F. M. Haggag et al., published by the American Society for Metals (ASM) in the Proceedings of the Fracture-Mechanism Program of the International Conference and Exposition on Fatigue, Corrosion Cracking, Fracture Mechanics and Failure Analysis, 2-6 Dec. 1985, Salt Lake City, Utah, pages 399-406, briefly describes this simulated testing approach as well as some of the problems in this area of testing.
Accurate determination of the flow properties and fracture toughness at the worst-damaged/deformed component location is essential for a complete and reliable fracture mechanics analysis in order to assure the safe operation of a metallic structure. Presently no other single in-field or laboratory device or technique exists that can obtain directly and accurately the wide range of mechanical and physical properties that the Field Indentation Microprobe (FIM) apparatus of this invention can measure. These properties include elastic (Young's) modulus, yield strength for all metallic materials including those exhibiting Luders (inhomogeneous) strains, Luders strain, strain-hardening exponent, actual material flow properties (true-stress/true-plastic-strain curve) up to 20% true-plastic-strain, fracture toughness, component thickness for determining allowable stress, thickness strains in deformed structural components, residual stress presence and orientation, crack identification and characterization, and creep. Furthermore, the mechanical properties can be measured at specified controlled strain rates and at a wide range of test temperatures, from a very small volume of the structure.
In contrast, some hardness test techniques such as the Rockwell hardness, or others using ball indenters, can be used to measure hardness only. Although sometimes these hardness numbers are converted to ultimate tensile strength values, such conversions are at best approximations and the American Society for Testing and Materials (Section 9.1 of ASTM Standard E 18-84) recommends that such conversions should be avoided. Furthermore, accurate conversion of such hardness numbers to yield-strength values, essential for fracture mechanics analysis and determination of a shift in the ductile-to-brittle transition temperature for steel plates and welds, can not be made. The FIM apparatus does not measure hardness, but instead measures a larger set of physical and mechanical properties, as will be explained later, and uses accurate and innovative techniques to reduce these measurements into meaningful information which is required to evaluate the integrity of the structure of interest.
The main problem in determining the yield strength from indentation tests is related to the Luders strain behavior. In a uniaxial tensile test the Luders strain is shown by the inhomogeneous plateau (horizontal portion) of the stress-strain curve and is confined mostly to a defined volume of the specimen gage section. Hence, the inhomogeneous (Luders) and homogeneous (work hardening) plastic behaviors in a tensile test are well defined and separated from each other. In contrast, in an indentation test both occur simultaneously throughout the test because the material has less constraint at the surface around the indentation. With increasing loads, an increasing volume of material is forced to yield and flow under multiaxial compression caused by the indenter, and more material pile-up and Luders strain occur around the indentation. Consequently, an accurate determination of the yield strength should be based on the entire load-displacement curve of the indentation test as explained later for the FIM testing. Luders strain behavior in ball indentation testing is discussed and demonstrated in an article co-authored by the inventor and entitled "Determination of Luders Strains and Flow Properties in Steels from Hardness/Microhardness Tests", published in Metallurgical Transactions A, Vol. 14A, pages 1607-1613 (August 1983). The technique reported in this article involved the use of a prior developed correlation (using either optical interferometry or mechanical profilometry techniques) between Luders strain and the geometry of the lip (material pile-up) around a ball indentation, in order to determine the Luders strain and then the yield strength for a certain carbon steel material. Since such a correlation for each material may not exist and its development is expensive, the FIM of this invention does not utilize such an approach. Furthermore, the use of optical interferometry is impractical for field testing application.
The present invention was conceived to incorporate innovative solutions to ten principal needs of in-situ nondestructive evaluation of structural integrity. The first is the in-field and nondestructive applicability. The second is the procedures which combine ease of field testing, simplicity, and computerized control of test procedure, data acquisition, and data analysis. The third is the automated measurement of material pile-up and geometry around the indentation. The fourth is the novel data acquisition/analysis techniques to measure, at a single location, the necessary material characteristics for subsequent property calculations. The fifth is the use of a very small material volume to measure properties nondestructively. The sixth is the adaptability to a wider variety of field and laboratory applications, e.g., weld inspection/qualification and new alloy evaluation. The seventh is the simulation of inservice structural loading conditions conveniently and more economically in laboratory or field. The eighth is the computer control of a motorized testing head to allow accurate positioning and use of adjacent ultrasonic transducers to determine material thickness and related changes caused by the testing procedure, crack presence and size, and presence and orientation of residual stresses. The ninth is the interchange to other appropriate indenter geometries for related types of testing such as indentation creep testing. The tenth is the use of both plastic and total (elastic plus plastic) indentation depth, as well as unloading curve slope, in test data analysis.
During the course of a preliminary patent search, the following patents were located: U.S. Pat. Nos. 2,158,008, issued to R. L. Grant, Jr. on May 9, 1939; 4,433,582, issued to M. W. Joosten on Feb. 28, 1984; German patentschrift No. 201-202; Russian Pat. No. 1185-248; and Japanese Pat. No. 58-196439.
Another reference that may be related to the present invention, due to computer control of testing using an indenter, is U.S. Pat. No. 4,621,523 issued to B. S. Shabel et al. on Nov. 11, 1986. In this patent the inventors thereof make the assumption that the diameter of impression does not change by elastic springback when the load on the indenter is removed (lines No. 11 and 12 of page 3 of the patent), thus ignoring the elastic recovery of the specimen. The applicant's approach in the present invention does not require this assumption. The patent of Shabel et al. further discusses obtaining hardness data at low and high strains and then relating the same to engineering yield and tensile strength values based upon empirical correlations between hardness and tensile data for a specific material. This material is aluminum in their case. Thus, the conversion of hardness data to engineering yield and tensile strength is not possible for unknown materials, new alloys, or welds. The FIM of this invention does not use such an approach or make such assumptions. The FIM of this invention measures the material flow properties by analyzing the elastic and plastic deformation during the cyclic loadings and unloadings of an indenter against a structure at the same indentation location. The analysis is based primarily on elastic and plastic theories. The FIM also accounts for the strain rate sensitivity of the test material while Shabel et al. do not. Indentation tests of the FIM are strain rate controlled via the use of an appropriately programmed computer and a data acquisition system, while Shabel's indentation tests are load controlled.
Shabel et al. do not take into account the fact that different materials behave very differently under uniaxial tensile loads. Some materials exhibit only a homogeneous plastic strain response, while others exhibit an initial non-homogeneous plastic behavior followed by a homogeneous response. The latter complex behavior is typical of most structural materials, such as carbon steels, aluminum alloys, titanium alloys, etc. Also, the material behavior under indentation tests will vary due to the presence of residual stresses that might result from accidental deformation or welding procedures. Relating hardness numbers from tests on these materials to yield strength values will not be successful.
In addition, Shabel et al. do not and can not measure flow properties (true-stress/true-plastic-strain curve up to 20% strain). The automated indentation test as disclosed and taught by the Applicant does measure these properties. The FIM of this invention provides a stress-strain diagram in which corresponding values of true-stress and true-plastic-strain are plotted against each other. The values of stress are plotted as ordinates (vertically) and values of strain as abscissa (horizontally). The true-stress/true-plastic-strain curve is particularly needed for the analysis of pipes and pressure vessels. Furthermore, the apparatus of Shabel et al. does not measure the elastic (Young's) modulus, strain-hardening exponent, Luders strain, and fracture toughness of test materials. The numerous mechanical and physical properties measured by the FIM of this invention are discussed below.
Two other references, which deal only with hardness testing, are U.S. Pat. Nos. 4,199,976 issued to J. C. Edward on Apr. 29, 1980; and 4,635,471 issued to D. B. Rogers et al. on Jan. 13, 1987. There is no provision in either of these references as to the determination of any property other than hardness. Edward does teach magnet means for attaching the hardness apparatus to the material being tested, and Rogers et al. teaches cleaning of test area of pipe prior to testing (as known by others).
Other references showing the general art of hardness testing using an indenter probe are U.S. Pat. Nos. 4,671,104 issued to H. Fisher on June 9, 1987; 3,822,946 issued to G. A. Rynkowski on July 9, 1974; 4,331,026 issued to B. S. Howard et al. on May 25, 1982; and 3,879,982 issued to E. Schmidt on Apr. 29, 1975. None of the devices described in these patents provide information other than hardness; none can provide the broad information which is determined by the present invention.
Accordingly, it is an object of the present invention to provide an in-the-field and substantially nondestructive Field Indentation Microprobe (FIM) apparatus so as to measure, from load/displacement data during both loading and unloading, the yield strength, flow properties (true-stress/true-plastic-strain curve), strain hardening exponent, Luders strain, elastic modulus, and fracture toughness of the concerned/actual component material, even when it is in a damaged, deformed, aged, or embrittled condition.
An additional object of the present invention is to provide a system that will determine the yield strength and the true-stress/true-plastic-strain curve for materials exhibiting only homogeneous (work hardening) strain behavior as well as those exhibiting both homogeneous and inhomogeneous (Luders) strain behavior.
It is another object of the present invention to provide appropriate testing procedures for use with the field indentation microprobe (FIM) whereby accuracy, computerized test control and data acquisition and analysis are achieved.
It is also an object of the present invention to provide an in-the-field testing apparatus that can provide output information as to a wide variety of mechanical properties based upon the cyclic application (and release) of increasing loads to an indenter in contact with the surface of interest at a single location.
Another object of the invention is to provide means for measuring the amount and geometry of material pile-up around the indentation created by the indenter of the FIM in order to determine the presence and orientation of residual stresses in metallic materials and, alternatively, Luders strain in materials such as low carbon steels, aluminum alloys, and titanium alloys, based on previously established correlations.
A further object is to provide apparatus which utilizes a very small surface area and volume of the structure under test and does not affect the integrity of that structure.
These and other objects of the present invention will become more apparent upon a consideration of the following drawings and a detailed description thereof.